Um. Right. You know, she's a very good actress when she's in good films with good directors. In fact, she's been winning award after award for Black Swan.
Everyone was bad in the prequels. Everyone. Yes, even Liam Neeson (he looks bored as shit the entire time) and Ian McDiarmid (in Episode III, anyway). McGregor got off lucky for episodes II and III, but he wasn't very good in episode I.
So yeah - a Star Wars prequel is not a very good indicator of acting ability.